R.D. Hewson

Geologic and alteration mapping at Mt Fitton, South Australia, using ASTER satellite-borne data

The Japanese ASTER sensor on board the US Terra satellite was launched in December 1999 to establish a spaceborne capability for high spatial, multispectral visible-shortwave infrared and thermal infrared remote sensing data mapping of the Earths environment. The Mt Fitton test site in South Australia was chosen to test the ability of the ASTER instrument for geological mapping having been previously surveyed by several visible-shortwave IR and thermal IR airborne remote sensing instruments and several field campaigns collecting relevant spectral measurements. These previous airborne remote sensing surveys and field campaigns successfully mapped a suite of intrusives and sedimentary units with some greenschist metamorphic and localised hydrothermal alteration. Visible-NIR ASTER channels successfully mapped green vegetation and iron oxide information. ASTER SWIR data were spectrally unmixed into four spectrally recognizable endmembers that relate to areas rich in talc, chlorite, white mica and carbonate mineralogies. This result was confirmed using IRIS field spectra resampled to ASTER resolution wavelengths. Quartz, carbonate and talc-tremolite rich units at Mt Fitton were also discriminated using ASTERs thermal infrared data. These results from low level ASTER data products indicated that ASTER could discriminate mineral groups not achievable from Landsat TM, though more precise mineral species mapping is not possible.

The performance of the satellite-borne Hyperion hyperspectral VNIR-SWIR imaging system for mineral mapping at Mount Fitton, South Australia

Satellite-based hyperspectral imaging became a reality in November 2000 with the successful launch and operation of the Hyperion system on board the EO-1 platform. Hyperion is a pushbroom imager with 220 spectral bands in the 400-2500 nm wavelength range, a 30 meter pixel size and a 7.5 km swath. Pre-launch characterization of Hyperion measured low signal to noise (SNR<40:1) for the geologically significant shortwave infrared (SWIR) wavelength region (2000-2500 nm). The impact of this low SNR on Hyperions capacity to resolve spectral detail was evaluated for the Mount Fitton test site in South Australia, which comprises a diverse range of minerals with narrow, diagnostic absorption bands in the SWIR. Following radiative transfer correction of the Hyperion radiance at sensor data to surface radiance (apparent reflectance), diagnostic spectral signatures were clearly apparent, including: green vegetation; talc; dolomite; chlorite; white mica and possibly tremolite. Even though the derived surface composition maps generated from these image endmembers were noisy (both random and column), they were nonetheless spatially coherent and correlated well with the known geology. In addition, the Hyperion data were used to measure and map spectral shifts of <10 nm in the SWIR related to white mica chemical variations.

Applicability of the Thermal Infrared Spectral Region for the Prediction of Soil Properties Across Semi-Arid Agricultural Landscapes

In this study we tested the feasibility of the thermal infrared (TIR) wavelength region (within the atmospheric window between 8 and 11.5 μm) together with the traditional solar reflective wavelengths for quantifying soil properties for coarse-textured soils from the Australian wheat belt region. These soils have very narrow ranges of texture and organic carbon contents. Soil surface spectral signatures were acquired in the laboratory, using a directional emissivity spectrometer (μFTIR) in the TIR, as well as a bidirectional reflectance spectrometer (ASD FieldSpec) for the solar reflective wavelengths (0.4–2.5 μm). Soil properties were predicted using multivariate analysis techniques (partial least square regression). The spectra were resampled to operational imaging spectroscopy sensor characteristics (HyMAP and TASI-600). To assess the relevance of specific wavelength regions in the prediction, the drivers of the PLS models were interpreted with respect to the spectral characteristics of the soils’ chemical and physical composition. The study revealed the potential of the TIR (for clay: R2 = 0.93, RMSEP = 0.66% and for sand: R2 = 0.93, RMSEP = 0.82%) and its combination with the solar reflective region (for organic carbon: R2 = 0.95, RMSEP = 0.04%) for retrieving soil properties in typical soils of semi-arid regions. The models’ drivers confirmed the opto-physical base of most of the soils’ constituents (clay minerals, silicates, iron oxides), and emphasizes the TIR’s advantage for soils with compositions dominated by quartz and kaolinite.

Airborne hyperspectral imaging of hydrothermal alteration zones in granitoids of the Eastern Fold Belt, Mount Isa Inlier, Australia

ABSTRACT Hyperspectral remote sensing data from the Eastern Fold Belt, Mount Isa Inlier, Australia were compared with petrographic and geochemical studies to map the spatial extension and compositional variations of Proterozoic granitoids and endoskarns as well as hydrothermal alteration patterns in adjoining metasedimentary successions. Detailed spatial analysis of spectral remote sensing data shows an almost circular alteration zoning in the Mallee Gap Granite, which was emplaced during a late phase of the Mesoproterozoic Williams event. A combination of hyperspectral images, such as white mica, kaolin and MgOH products, were used to map the alteration zoning. The formation of the endoskarn is presumably related to autometasomatism and interaction with fluids released from the country rocks during a late phase of the emplacement. The intrusion of the Mallee Gap Granite has only a local control on the hydrothermal alteration, but high potassic granites of the southern Mount Angelay Granite might have expelled oxidized mineralizing fluids and possibly had a major impact on regional scale alteration. Hyperspectral remote sensing data may be used to estimate the imprint of single igneous bodies on the Mesoproterozoic hydrothermal evolution of the Eastern Fold Belt and are important for the study of ore-forming hydrothermal processes in general.

Satellite-derived mineral mapping and monitoring of weathering, deposition and erosion

The Earth’s surface comprises minerals diagnostic of weathering, deposition and erosion. The first continental-scale mineral maps generated from an imaging satellite with spectral bands designed to measure clays, quartz and other minerals were released in 2012 for Australia. Here we show how these satellite mineral maps improve our understanding of weathering, erosional and depositional processes in the context of changing weather, climate and tectonics. The clay composition map shows how kaolinite has developed over tectonically stable continental crust in response to deep weathering during northwardly migrating tropical conditions from 45 to 10 Ma. The same clay composition map, in combination with one sensitive to water content, enables the discrimination of illite from montmorillonite clays that typically develop in large depositional environments over thin (sinking) continental crust such as the Lake Eyre Basin. Cutting across these clay patterns are sandy deserts that developed <10 Ma and are well mapped using another satellite product sensitive to the particle size of silicate minerals. This product can also be used to measure temporal gains/losses of surface clay caused by periodic wind erosion (dust) and rainfall inundation (flood) events. The accuracy and information content of these satellite mineral maps are validated using published data.

Investigations into Soil Composition and Texture Using Infrared Spectroscopy (2–14 m)

The ability of thermal and shortwave infrared spectroscopy to characterise composition and texture was evaluated using both particle size separated soil samples and natural soils. Particle size analysis and separation into clay, silt, and sand-sized soil fractions was undertaken to examine possible relationships between quartz and clay mineral spectral signatures and soil texture. Spectral indices, based on thermal infrared specular and volume scattering features, were found to discriminate clay mineral-rich soil from mostly coarser quartz-rich sandy soil and to a lesser extent from the silty quartz-rich soil. Further investigations were undertaken using spectra and information on 51 USDA and other soils within the ASTER spectral library to test the application of shortwave, mid- and thermal infrared spectral indices for the derivation of clay mineral, quartz, and organic carbon content. A nonlinear correlation between quartz content and a TIR spectral index based on the 8.62 μm was observed. Preliminary efforts at deriving a spectral index for the soil organic carbon content, based on 3.4–3.5 μm fundamental H–C stretching vibration bands, were also undertaken with limited results.

Mapping porphyry-skarn alteration at Yerington, Nevada, using airborne hyperspectral VNIR-SWIR-TIR imaging data

The alteration mineralogy of a porphyry-skarn system near Yerington, Nevada, was mapped using combined airborne hyperspectral HyMap and SEBASS data. The VNIR-SWIR HyMap data provided information about the abundance and levels of Tschermak substitution in white micas, as well as mapping the Mg-Fe chemistry of chlorite. The TIR SEBASS data provided unique information about the abundance and Fe-Al chemistry of garnet and Na-Ca chemistry of plagioclase feldspar. Both wavelength regions mapped calcite, dolomite, amphibole and epidote. The derived mineral maps were validated through spectral, mineralogical and chemical analyses of associated field samples. From these mineral maps, the geometry and physicochemistry of porphyry-skarn hydrothermal system becomes apparent, including the sites for Cu mineralization.

Mapping variations in plagioclase felspar mineralogy using airborne hyperspectral TIR imaging data

The Na-Ca chemistry of plagioclase felspar was measured and mapped remotely using airborne hyperspectral thermal infrared (TIR) SEBASS image data collected from a well-exposed porphyry system near Yerington, Nevada. This followed correction of the airborne SEBASS data for instrument and atmospheric effects and temperature-emissivity separation and was validated through associated field and laboratory studies. The later found a linear correlation between the relative intensities of the plagioclase /spl mu/FTIR emissivity spectral features at 9.6 and 10.0 /spl mu/m and the XRD dspacing of the felspar 111/130 hkl reflection, both of which are sensitive to the Na-Ca chemistry of plagioclase. The SEBASS derived felspar mineralogy maps clearly show variations in Na-Ca plagioclase felspar chemistry related to the nature of the host rock and any superimposed hydrothermal alteration.

Using the Geoscience Australia-CSIRO ASTER maps and airborne geophysics to explore Australian geoscience

This study evaluated the geological mapping potential of the recently released Australian CSIRO-GA ASTER satellite geoscience products in providing mineral abundance and compositional information. A range of environments was examined by using test sites including the temperate cultivated New South Wales area of Wagga Wagga, and the semi-arid rangeland Mt Fitton of South Australia. Data integration of the ASTER derived products was undertaken with geophysical data, digital elevation models and fractional vegetation cover information. The study demonstrated that these products can successfully assist geological mapping within semi-arid areas and, to a lesser extent, within temperate open woodland environments.

Mapping Soil Surface Mineralogy at Tick Hill, North-Western Queensland, Australia, Using Airborne Hyperspectral Imagery

The use of airborne hyperspectral imagery for mapping soil surface mineralogy is examined for the semi-arid Tick Hill test site (20 km2) near Mount Isa in north-western Queensland. Mineral maps at 4.5 m pixel resolution include the abundances and physicochemistries (chemical composition and crystal disorder) of kaolin, illite–muscovite, and Al-smectite (both montmorillonite and beidellite), as well as iron oxide, hydrated silica (opal), and soil/rock water (bound and unbound). Validation of these hyperspectral mineral maps involved field sampling (34 sites) and laboratory analyses (spectral reflectance and X-ray diffraction). The field spectral data were processed for their mineral information content in the same way as the airborne HyMap data processing. The results showed significant spatial and statistical correlation. The mineral maps provide more detailed surface composition information compared with the published soil and geological maps and other geoscience data (airborne radiometrics and digital elevation model). However, there is no apparent correlation between the published soil types (i.e. Ferrosols, Vertosols, and Tenosols) and the hyperspectral mineral maps (e.g. iron oxide-rich areas are not mapped as Ferrosols and smectite-rich areas are not mapped as Vertosols). This lack of correlation is interpreted to be related to the current lack of spatially comprehensive mineralogy for existing regional soil mapping. If correct, then this new, quantitative mineral-mapping data have the potential to improve not just soil mapping but also soil and water catchment monitoring and modelling at local to regional scales. The challenges to achieving this outcome include gaining access to continental-scale hyperspectral data and models that link the surface mineralogy to subsurface soil characteristics/processes.